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In Reply to: RE: High transconductance tubes sound as transistors... posted by Paul Joppa on March 30, 2014 at 17:46:10
"Unfortunately, tubes and circuits that use them are - at least a little - nonlinear EVERYWHERE. So for this to be meaningful, there must be some arbitrary boundary between "highly linear" and "not so linear"."
I guess I shouldn't be surprised when I make absolute statements and people take me seriously. I wasn't trying to be specific, I was really just speaking in general.
All I was really trying to say was there is no excuse for running triodes in very non-linear ways.
I was in a Chinese 300b amp the other day and the first stage was a 6sn7 running at about 3.4ma with a 62k plate resistor with 70 volts on the plate direct coupled to another 6sn7.
Or how about a 7b4 running less than .5ma with a 285k plate resistor at about 210 volts plate trying to drive an output tube with about 70pf of Miller capacitance.
Swinging the grid just 1.5 volts peak to peak causes a -55 and a +43 volt plate change.
Neither of those are what I would call highly linear operating points.
In fact they are both butt ugly.
With the 7b4 and a CCS plate load at 1.2ma and 200 volts plate, we can get -48 and +48 plate change from a 1 volt peak to peak grid swing.
I call that linear. (It still won't drive the 70pF though)
Oops, there I go again, making absolute statements that someone's going to parse and rip apart.
Oh well, I have broad shoulders and thick skin. I can take it.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Using a Pimm-style mu-follower CCS could probably deal with the 70-peeks of grid capacitance.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Thanks, man. Yeah, I was expressing some frustration of my own - I apologize if the flak hit you accidentally. :^)
But I must say, you brought up a serious and important question, which it was my intention to further address - what (exactly, measurably) is a good definition of linear operation?
My own rule of thumb for power triodes - I can't call it a definition - is to design for a load line that gives twice the quiescent current, at zero grid-cathode volts. With a really linear tube (see below) that gives about 3% second harmonic at full output. Less linear tubes will have more distortion, I've calculated as much as 10% based on published curves, but those tubes will distort at practically any operating point! The reason I chose this is that even with huge negative feedback the tube will remain in Class A throughout a full sinewave cycle.
My model of tube linearity is based on a distribution of mu along the cathode. If the mu is everywhere the same, then cutoff (zero plate current) happens at the same grid voltage everywhere. I call such a triode "linear" because it follows the 3/2 power law exactly. That law is of course not a linear function, but you can't in theory do any better with a triode.
Of course, if the load impedance is much greater than that which produces the most power - which is the case for drivers if they have a high-impedance plate load such as a choke or current source - then it is easier to operate the tube in a region where the mu does not vary significantly, and you can this way approach truly linear operation.
This leads to the idea that there can be a region of nearly constant mu, within which tube operation is actually quite linear. But you still need a number - perhaps a 10% variation of mu is acceptable? Open for comments!
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